This invention relates to oxygen sensing, and more specifically to the quantitative measurement of oxygen concentration in a gaseous mixture, using a zinc-air cell. The invention has utility in a broad range of applications, including general anaesthesia in surgery, mine safety, etc.
It has been discovered that zinc-air cells, which are in common use as xe2x80x9chearing aid batteries,xe2x80x9d can effectively measure oxygen concentration in a gaseous mixture, and that, with appropriate circuitry, accurate quantitative measurements of oxygen concentration can be achieved over substantially the full range from 0% to 100%. In an apparatus described in U.S. Pat. No. 6,099,707, granted Aug. 8, 2000, and developed by Doxs Technology Systems, Inc. of 2090 Bondsville Road, Downingtown, Pa. 19335, the voltage across the terminals of a zinc-air oxygen sensing cell is held at a constant level, and the current in the cell is measured. With a proper choice of the constant voltage level across the cell, e.g. 0.6 volts, the current in the cell can be made to vary monotonically with oxygen concentration over the full range from 0% O2 to 100% O2, so that a direct meter reading of oxygen concentration can be achieved over that range.
The Doxs Technology Systems apparatus has the unique advantage that conventional zinc-air cells are relatively inexpensive and disposable. However, the Doxs apparatus also has certain disadvantages. It utilizes two operational amplifiers, one to establish a fixed voltage level at one terminal of the zinc-air cell, and the other both to maintain a fixed voltage level at the other terminal of the cell and to serve as a current-to-voltage converter, providing an output voltage proportional to the cell current. To do this, the operational amplifiers need to be able to deliver relatively high currents, and consequently consume electrical power at levels requiring large battery power supplies or frequent battery replacement.
The output current of a typical zinc-air cell varies from 0 ma. at 0% oxygen concentration to well over 90 ma. at 100% oxygen concentration. Optimization of the level of the voltage maintained across the cell for stable and accurate operation at low O2 concentrations results in high cell and operational amplifier currents at high O2 concentrations. Conversely, optimization of the voltage maintained across the cell for stable, accurate operation at high O2 concentrations, can result in unstable and unpredictable operation at low O2 concentrations, and at these low O2 concentrations, operational amplifier operating current is still a concern.
Another difficulty with the prior apparatus is that, when the zinc-air cell is first installed in the measuring circuit, it produces large currents as the nominal 1.4 volt potential difference between its terminals changes to the imposed potential difference, which is typically 0.6 volts. During this time the current produced by the cell can exceed 100 ma. The circuit components capable of handling currents at this level are expensive, energy-consuming and the their power requirements are such that they are not well-suited for portable instruments. Modifications enabling the circuit to reduce the potential difference across the cell terminals from 1.4 to 0.6 volts without drawing large amounts of current introduce an excessive time delay between the installation of the zinc-air cell and the time at which the instrument is ready for operation.
Still another difficulty with the prior apparatus is that the relationship between cell current and oxygen concentration is non-linear, the current rising exponentially with increasing oxygen concentration. This makes it difficult to translate cell current to a digital output providing an O2 concentration reading. It also results in a shortening of the useful life of the zinc-air cell especially when used at high oxygen concentrations.
A general object of the invention is to provide an improved apparatus for measuring oxygen concentration, which is inexpensive to manufacture and operate, which provides reliable measurements, and which is better suited for portable operation. Another object of the invention is to provide an apparatus capable of measuring oxygen concentration over a wide range but requiring only a small operating current. Another object of this invention is to provide an improved apparatus which provides stable and predictable operation over a long time. Still another object is to measure oxygen concentration over a wide range while producing an analog output signal which is a nearly linear representation of oxygen concentration, and which is therefore readily converted to digital format by an analog-to-digital converter.
The apparatus in accordance with the invention utilizes a sensor comprising a pair of sensor terminals having one or more zinc-air cells connected between them. Preferably the sensor comprises only a single zinc-air cell. Each zinc-air cell in the sensor has a pair of electrodes adapted to be exposed to a gaseous mixture in which the oxygen concentration is to be measured. The sensor is connected into a circuit for automatically maintaining the potential difference between the sensor terminals within a range below the open-circuit voltage across the sensor terminals when the sensor is exposed to air, i.e. to a gaseous mixture containing 20.9% oxygen. The circuit comprises a shunt circuit branch connected across the sensor terminals. This shunt circuit branch includes a transistor arranged to establish a controlled load on the sensor. The transistor is preferably a field-effect transistor having its source-drain circuit in the shunt circuit branch. An output circuit is connected to the sensor for delivering an electrical output signal proportional to the current in the sensor. Preferably the shunt circuit branch includes a resistor in series with the output current terminals of the transistor, i.e. the source and drain in the case of a field-effect transistor. The resistor and one of the transistor terminals are connected at a junction, and the output circuit is connected to the junction.
A control circuit, connected in driving relationship to the transistor, preferably includes means for adjusting the drive on the transistor and thereby adjusting the load on the sensor, and also preferably includes a feedback loop connected to sample the current in the sensor and to increase the potential difference between the electrodes of each cell of the sensor, by reducing the load on the sensor, as the sensor current increases, thereby increasing the linearity of the relationship between the electrical output signal and the oxygen concentration in the gaseous mixture.
The circuit, especially when utilizing a field-effect transistor as the active element of the shunt circuit branch, requires a minimum of operating current and provides an analog output varying monotonically with oxygen concentration over a wide range, which can extend from 0% to 100% oxygen. The feedback loop keeps the zinc-air cell current within its ordinary operating range, thereby yielding stable, predictable long-life operation. The feedback loop also provides an analog output which is more nearly linearly related to oxygen concentration over a wide range, simplifying conversion of the analog output signal to a digital output directly representing oxygen concentration.
Modifications can be made to the sensing cell to make it responsive to other gases such as nitrogen and carbon monoxide. Thus, although the circuit of the invention finds its principal application in the measurement of oxygen concentration, it also has application in the measurement of concentrations of such other gases.